Forwarding Path With .htaccess - .htaccess

Question

I use cPanel hosting on https://files.example.com and I'd like https://files.example.com/anything-here to redirect to my main website and forward the path, so you'd end up on https://www.example.com/anything-here. Is this possible?
Note that I also have a forced SSL redirect inside my .htaccess file.
https://www.example.com is a Google Site.
My .htaccess file:
ErrorDocument 400 /index.html
ErrorDocument 401 /index.html
ErrorDocument 403 /index.html
ErrorDocument 404 /index.html
ErrorDocument 500 /index.html
RewriteEngine On
RewriteCond %{HTTPS} !=on
RewriteRule ^(.*)$ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301,NE]
Header always set Content-Security-Policy "upgrade-insecure-requests;"

Answer 1

https://www.example.com is a Google Site.
If the two sites are on different servers and you simply need to redirect everything from one host to the other and maintain the same URL-path then you don't appear to need anything in your .htaccess file at files.example.com except for the following mod_alias Redirect directive:
# Redirect everything to https://www.example.com/ and maintain URL-path
Redirect 302 / https://www.example.com/
The Redirect directive is prefix matching and everything after the match is copied onto the end of the target URL. So, a request for /foo is redirected to https://www.example.com/foo.
If, however, you have other hostnames pointing to your cPanel account then you'll need to use a mod_rewrite rule and check the requested hostname.
For example, at the end of your existing .htaccess file:
# Redirect every request for "files.example.com"
# to https://www.example.com/ and maintain URL-path
RewriteCond %{HTTP_HOST} ^files\.example\.com [NC]
RewriteRule ^ https://www.example.com%{REQUEST_URI} [R=302,L]
Reference:
https://httpd.apache.org/docs/2.4/mod/mod_alias.html#redirect
https://httpd.apache.org/docs/2.4/mod/mod_rewrite.html#rewriterule
UPDATE#1:
But I just realised that it's also forwarding the path for files that do exist on my hosting. I onlt want it to forward invalid paths through to www.example.com.
In that case, you'll need to do it like this instead:
RewriteCond %{HTTP_HOST} ^files\.example\.com [NC]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^ https://www.example.com%{REQUEST_URI} [R=302,L]
The 2nd and 3rd conditions check that the request does not map to an existing file (or directory) before issuing the redirect.
Remove the first condition that checks the HTTP_HOST if it's not required.
UPDATE#2:
Is there a way to have it exclude "/URL" from this? If "URL" is specified in the path (example.com/URL/whatever) then I do not want this .htaccess rule to take place. I just want it to use my ErrorDocuments for this path.
If it's just the one pattern you want to exclude, ie. all URLs that start /URL then you can modify just the RewritRule directive. For example:
:
RewriteRule !^URL https://www.example.com%{REQUEST_URI} [R=302,L]
Any URLs that do not start /URL will be excluded. Note that this also includes /URLwhatever, not just /URL/whatever.